Hydrogen is frequently cited to be the world's next generation fuel, since its oxidation does not emit greenhouse gases that contribute to climate change. Most major automakers are investing significantly in the development and commercialization of hydrogen vehicles. Other applications and uses of hydrogen include other transportation modes (trains, ships, utility vehicles, and so forth), as well as industries such as petrochemicals, oil sands upgrading, ammonia for fertilizer production, and others.
The predominant existing hydrogen production processes such as steam-methane reforming (SMR) use fossil fuels, so there is a need for a clean, reliable, safe, efficient and economic process for the production of hydrogen.
A known method is called the thermochemical copper-chlorine (Cu—Cl) cycle, which consists of a sequence of chemical and physical processes that decompose water into hydrogen and oxygen (see Table 1) [See Naterer, G. F., Suppiah, S., Stolberg, L., Lewis, M., Wang, Z., Daggupati, V., Gabriel, K., Dincer, I., Rosen, M. A., Spekkens, P., Lvov, L., Fowler, M., Tremaine, P., Mostaghimi, J., Easton, E. B., Trevani, L., Rizvi, G., Ikeda, B. M., Kaye, M. H., Lu, L., Pioro, I., Smith, W. R., Secnik, E., Jiang, J., Avsec, J., “Canada's Program on Nuclear Hydrogen Production and the Thermochemical Cu—Cl Cycle”, International Journal of Hydrogen Energy, vol. 35, pp. 10905-10926, 20101.
TABLE 1Steps of chemical reactions in the copper-chlorine cycleTemperatureFeedStepReactionRange (° C.)°Output*1Electrolysis:<100Feed:Aqueous CuCl and HCl + V2CuCl(aq) + 2HCl(aq) →Output:H2 + CuCl2 (aq)H2(g) +2CuCl2(aq)2Concentrating:<100Feed:Slurry containing HCl and CuCl2 + QCuCl2(aq) → CuCl2(s)OutputGranular CuCl2 + H2O1HCl vapours3Hydrolysis:400Feed:Powderlgranular CuClz + H20(g) + Q2CuCl2(s) + H20(g) →Output:Powderlgranular CuO*CuCl2 + 2HCl (g)CuO*CuCl2(s) + 2HCl(g)4Thermolysis:500Feed:Powderlgranular CuO*CuCl2(s) + QCuO*CuCl,(s) → 2CuCl(I) +Output:Molten CuCl salt + oxygen1/202(g)*Q = thermal energy, V = electrical energy
Naterer et al. (2010) have outlined advances in the Cu—Cl cycle, particularly with respect to hydrogen production with Canada's Generation IV reactor, called SCWR (Super-Critical Water Reactor). Other heat sources may also be utilized for the Cu—Cl cycle, such as solar energy or industrial waste heat.
Another method is a thermochemical magnesium-chlorine-sodium/potassium-CO2 (Mg—Na/K—Cl—CO2) cycle, which consists of a sequence of chemical and physical processes that decompose water into hydrogen and oxygen (see Table 2) and at the same time capture and purify CO2.
TABLE 2Steps of chemical reactions in a new Mg—Cl—K/Na—CO2 cycle OutputTemperatureFeed/StepReactionRange (° C.)OutputAElectrolysis:<100Feed:Aqueous NaCl + V/2NaCl(aq) + 2H20(1) + V =(electrolysis)Output:Aqueous NaOH + gasesou Cl2 and H22NaOH(aq) + Cl2(g) + H2(g)B-100Carbonate formation:<100Feed:Aqueous NaOH + gaseous CO2/2NaOH(aq) + CO2(g) =Output:aqueous Na2CO3Na2CO3(aq) + H20(1) + QB-101Carbonate formation:<100Feed:Solid NaOH + gaseous CO2/2NaOH(s) + CO2(g) = Na2C03(s) +Output:Solid Na2C03H20(g) + QB-11Bicarbonate formation:<100Feed:Aqueous NaOH + gaseous CO2/NaOH(aq) + CO2(g) =Output:Precipitated NaHC03NaHC03(s) + QB-12Na2CO3(s) + CO2(g) + H20(g) =<200Feed:Solid Na2C03, gaseous CO2 and H20/2NaHC03(s) + QOutput:Solid NaHC03B-13Na2C03(aq) + CO2(g) + H20(1) =<200Feed:Aqueous Na2C03, gaseous CO2 and2NaHC03(s) + Qliquid H20/Output:Solid NaHC03B-2Carbonate release:<200Feed:Solid Na2C032NaHC03(s) + Q = Na2C03 (s)Output:Gaseous CO2 and H20CPrecipiatation of MgC03:300-400Feed:Aqueous Na2CO3 and MgCl2/Na2C03(aq) + MgCl2(aq or s) →Output:Solid MgC03 and aqueous NaClMgC03(s) + 2NaCl (aq)DCalcination300-600Feed:Solid MgC03 + Q/MgC03(s) + Q = MgO(s) +: CO2(g)Output:Solid MgO + gaseous CO2EO2 production:300-600Feed:Solid MgO + gaseous Cl2 + Q/MgO(s) + Cl2(g) + Q = MgCl2(s) +Output:Solid MgCl2 + gaseous O21/202(g)Q = thermal energy,V = electrical energySodium (Na) element can be replaced with potassium (K)
It has been well documented that carbon dioxide emissions to the atmosphere from fossil fuels are contributing to climate change. The post-combustion technologies of capturing and sequestering carbon dioxide in the ground are very expensive and subject to a number of technical and other challenges, including the challenge of relatively small CO2 levels due to the presence of N2, and the uncertainty about whether or not the captured CO2 will indeed remain underground for a prolonged period, removal of oxygen from the atmosphere to the ground, legal liabilities of potential leakage, etc.
A widely studied approach to reducing CO2 emissions is CO2 capture at a power plant, transport by pipeline to an underground injection site, and sequestration for long-term storage in a suitable geologic formation. [See Figueroa, J. D., Fout, T., Plasynski, S., McIlvried, H., “Advances in CO2 capture technology—The U.S. Department of Energy's Carbon Sequestration Program”, International Journal of Greenhouse Gas Control, vol. 12, pp. 9-20, 2008]. CO2 capture from thermal power plants can be achieved in various ways: post-combustion capture, pre-combustion capture, and oxy-combustion. The relevant technologies for separation techniques include gas phase separation, absorption into a liquid, adsorption on a solid, hybrid adsorption/membrane systems, metal organic frameworks, ionic liquids, and enzyme-based systems. Other emerging concepts are described by Yang at al. [See Yang, H., Xu, Z., Fan, M., Gupta, R., Slimane, R. B., Bland, A. E., Wright, I., “Progress in carbon dioxide separation and capture: A review”, Journal of Environmental Science, vol. 20, pp. 14-27, 2008], including chemical-looping combustion and hydrate-based separation.
An alternative to carbon recycling in the technical literature is capturing of carbon dioxide in the atmosphere by first capturing CO2 and then combining it with H2 to produce useful products such as organic chemicals, materials, synthetic fuels or carbohydrates (see examples below).CO2+H2→CO+H2O; H298K=41.2kJ mol−1(production of carbon monoxide)  (1)CO2+4H2→CH4+2H2O; H298K=−252.9kJmol−1(production of methane)  (2)CO2+3H2→CH3OH+H2O; H298K=−49.5kJ mol−1(production of methanol)  (3)CO2+H2→HCOOH; G273K=32.9 kJ mol−1 (production of formic acid)  (4)
Hydrogenation of carbon dioxide is an alternative to underground sequestration, as it represents a form of chemical recycling of carbon dioxide to other useful forms such as methanol and dimethyl ether [See Olah, G. A., Goeppert, A., Prakash, G. K. S., “Chemical recycling of carbon dioxide to methanol and dimethyl ether: From greenhouse gas to renewable, environmentally carbon neutral fuels and synthetic hydrocarbons”, Journal of Organic Chemistry, vol. 74, no. 2, pp. 487-498, 2009]. CO2 recycling to methanol is the basis of a “methanol economy” described by Olah et al. (2009). Methods to convert CO2 to methanol include various catalytic and electrochemical conversion techniques. Methanol is a potentially viable transportation fuel for internal combustion engines (ICE) and fuel cells, as well as useful feedstock material for the production of synthetic hydrocarbons and their varied products. Recent developments in catalytic reactivity and reactor design of CO2 hydrogen processes have been described by Wang et al. [See Wang, W., Wang, S., Ma, X., Gong, J., “Recent advances in catalytic hydrogenation of carbon dioxide”, Chemical Society Review, vol. 40, pp. 3703-3727, 2011].
If the sources of hydrogen and electricity generation to drive the processes are clean and sustainable, then a carbon-neutral cycle can potentially be achieved. In other words, each carbon and water molecule would be recycled over and over again, thereby not contributing to a net accumulation of carbon dioxide in the atmosphere. For example, carbon dioxide from the air and hydrogen from water would be used to produce methanol, which is a Fuel that burns to release carbon dioxide, which is again captured and recycled.